The Raman effect refers to a situation where a sample is illuminated with one wavelength of light and a portion of the light is inelastically scattered, i.e. photons are emitted with a different wavelength than the incident photons. The Raman process occurs when the sample can absorb or emit a photon corresponding to the difference in energy between the incident and scattered beam. Spectroscopic measurements can be performed by measuring the amount of light scattered as a function of the difference in energy between the incident and scattered photons. This energy shift, the “Raman shift” is usually reported in wavenumbers (cm-1) that is the reciprocal of wavelength. Plotting the amplitude of scattered light versus the Raman shift allows the generation of vibrational spectra of materials. The positions of peaks in the Raman spectra can provide information about the structural, chemical and electronic properties of a sample.
Conventional Raman spectroscopy is a technique where a sample is illuminated by a pump beam and spectroscopic information about the sample is obtained by measuring wavelength-shifted light scattered from the sample. The scattered light may be at lower energy (Stokes shift) or higher energy (Anti-Stokes). Raman scattering can occur when a vibrational resonance in a sample can absorb or emit energy corresponding to the difference in frequency between the pump beam and the Raman scattered beam. In conventional Raman spectroscopy, the scattered beam is measured spectroscopically to determine the amount of scattered light as a function of between the pump beam and the scattered beam. Conventional Raman spectroscopy has been growing dramatically in recent years because of the rich information that can be obtained about molecular and electronic resonances in a wide variety of materials. It has also been growing relative to infrared spectroscopy as it has superior spatial resolution due to the smaller spot size attainable with visible, UV, or near IR wavelength illumination sources.
Raman spectroscopy does have some limitations, however. First, the Raman effect is often very weak. For example, most photons are scattered by the sample with no wavelength shift and roughly only 1 photon in 107 are shifted. Thus it is necessary to measure a small fraction of Raman shifted photons against a large background. This has led to the use of high power lasers that can easily damage samples. The Raman effect can also be obscured by sample fluorescence, another mechanism that products wavelength shifted photons. Finally, the spatial resolution is also still much coarser than that of a high-resolution technique such as probe microscopes. While some suppliers and publications claim better numbers, practical resolutions obtained by the typical user are around 1 μm or worse.
A variety of techniques have been developed to overcome the spatial resolution limit and increase signal strength. Many of these techniques rely on achieving plasmon amplification around a nanostructure. Such techniques are part of the reason that Raman Spectroscopy is currently in a growth mode, as nanotechnology tools are incorporated to improve Raman detection. One such technique, related to probe microscopy is called Tip-Enhanced Raman Spectroscopy. This technique illuminates a sample with a pump beam in the UV, visible, or near infrared and then detects wavelength shifted photons. This illumination takes place at the tip of a probe microscope, and the tip provides electrostatic (“lightning rod effect” and/or plasmon amplification at its apex. If the amplification is sufficient, a measurement of the Raman effect only in the vicinity of the tip can be separated from the background. The TERS effect has shown substantial promise in some key laboratories, but has not yet to be broadly adopted due to difficulties in reliably obtaining large enhancement factors and high spatial resolution.
Stimulated Raman
Stimulated Raman is a two photon technique that is used to enhance the sensitivity of conventional Raman spectroscopy and microscopy to measure and map Raman absorption bands in the mid-infrared. The basic concept is that two coherent laser beams at different wavelengths (or equivalently, two different optical frequencies) are incident on the same region of a sample. These two beams, called the pump beam and the Stokes beam, respectively, are arranged such that the difference in photon energies of the two beams can probe molecular vibrations in a sample under study. More specifically, the difference between the photon energies can be adjusted to sweep through and excite various molecular resonances. For example if the pump beam has an optical frequency of σ1 and the Stokes beam has a frequency σ2 energy can be efficiently absorbed by the sample with the difference frequency Δσ=σ1−σ2, corresponding to a molecular resonances of the sample. The molecular resonance are traditionally detected by measuring the amount of light transmitted, reflected, and/or scattered by a sample using an optical detector in the far field. Measuring the amount of light scattered by a sample as a function of the frequency difference between the two beams, a Raman absorption spectrum of a sample can be created.
Stimulated Raman achieves a significant enhancement over spontaneous Raman by combining the Raman effect with stimulated emission.
Stimulated Raman scattering generates dramatic enhancement in the intensity of the scattered Stokes beam versus spontaneous Raman scattering. The enhancement comes from the pump beam producing a large population of oscillators in an excited state and the Stokes beam enhancing the probability of the excited states decaying through emission of a photon at the Stokes frequency. (This effect is analogous to the stimulated emission and amplification that occurs in a laser.) The net effect is that the probability of emission of a Stokes photon can be dramatically increased versus spontaneous Raman. Gains of order 107 or more can be achieved allowing for efficient Raman imaging. At the same time, this mechanism also dramatically increases the likelihood of absorption of mid-IR photons at the difference wavenumber Δσ=σ1−σ2, where σ1 and σ2 are the wavenumbers of the pump and stokes beams. The stimulated Raman effect has been used for dramatic results recently in optical microscopy, for example real-time video imaging of biological samples by the Xie group at Harvard.
AFM-Based Stimulated Raman
A paper was published recently by I. Rajapaksa, and H. Kumar Wickramasinghe called “Raman Spectroscopy and Microscopy Based on Mechanical Force Detection.” This paper uses two narrowly tunable CW laser diodes in combination with a heterodyne detection techniques to create a tip-sample force that is proportional to the Raman absorption cross section of a material. The authors demonstrated the ability to obtain spectra over a narrow frequency region in the mid-infrared using two narrowly tunable diode lasers operating in the visible. The mechanism of detection is related to measuring the induced force between the tip and sample from polarization force originating from the electric fields of the incident laser beams interacting with the tip and sample. This paper showed the ability to obtain Raman spectra over a narrow region of the mid-IR and the ability to obtain force-based Raman contrast from individual Raman active molecules. Because of the nature of the sources used, and the experimental set-up, the Authors avoided any significant heating of the sample and restricted their measurements to force interactions due to electromagnetic effects.
Stimulated Raman Photoacoustic Imaging
Recent papers by the Scully group have stimulated Raman spectroscopy using an ultrasonic sensor to measure acoustic waves resulting from the absorption of pulses of Raman photons. They demonstrate spatially resolved stimulated Raman imaging with spatial resolution on the scale of 50-100 um. The acoustic waves are generated due to the absorption of IR energy in the Stimulated Raman process that leads to heating and rapid thermal expansion of the sample. As shown in U.S. Pat. No. 8,001,830, its CIP U.S. application Ser. No. 12/315,859, and other family members, all of which are incorporated by reference, the atomic force microscope can be used to measure and map rapid sample expansion due to optical absorption and sample heating. This technique and its variants is called Photo-Thermal IR Spectroscopy, or PTIR. The PTIR technique has a significant advantage over other photoacoustic techniques since it allows mapping of optical absorption induced thermal expansion on the nanoscale. While conventional photoacoustic techniques have spatial resolution that is fundamentally limited by optical diffraction (and in practice often much coarses), the AFM-based PTIR technique can use the tip of the AFM to measure thermal expansion on lateral scales far below the diffraction limit. Alternately, temperature sensing AFM probes (for example thermocouple or thermistor probes) can directly measure the temperature increase in the vicinity of the probe tip.
No technique, however, has provided robust, high resolution Raman spectroscopy over a wide range of Raman frequencies with spatial resolution consistently on the sub-micron scale.